This invention was made with Government support under contract NAS9 97023 awarded by NASA. The Government has certain rights in this invention.
1. Field of the Invention
This invention relates to the production and delivery of ozone in highly concentrated forms, both in high weight percent gas and high levels of ozone dissolved in water. More specifically, the invention relates to an electrochemical system capable of efficiently generating even small amounts of ozone.
2. Background of the Related Art
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removed cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. Ozone has also been shown to be useful in both gas and aqueous phase oxidation reactions which may be carried out by advanced oxidation processes (AOPs) in which the formation of OHxe2x80xa2 radicals is enhanced by exposure to ultraviolet light. Certain AOPs may even involve a catalyst surface, such as a porous titanium dioxide photocatalyst, that further enhances the oxidation reaction. There is even evidence that ozone will destroy viruses. Consequently, it is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. Ozone may also be employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. For many reasons, ozone is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone through the following reaction:
3/2O2xe2x95x90O3; xcex94Hxc2x0298=34.1 kcal
Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O3 by weight. Such O3 concentrations, while quite poor in an absolute sense, are still sufficiently high to furnish usable quantities of O3 for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful NOx otherwise known as nitrogen oxides. Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O3.
However O3 may also be produced by the electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte, i.e., electrically conducting, fluid. The electrolyte includes water, which, in the process dissociates into its respective elemental species, O2 and H2. Under the proper conditions, the oxygen is also evolved as the O3 species. The evolution of O3 may be represented as:
3H2Oxe2x95x90O3+3H2; xcex94Hxc2x0298=207.5 kcal
It will be noted that the xcex94Hxc2x0 in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high yields have not been realized in any forseeably practical electrolytic system.
The evolution of O3 by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O2/O3 gases evolved at the anode are comprised of 35% O3 by weight. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about xe2x88x9230xc2x0 C. to about xe2x88x9265xc2x0 C. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation.
Ozone, O3, is present in large quantities in the upper atmosphere in the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes, is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known.
Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl (OHxe2x80xa2) and peroxyl (HO2xe2x80xa2) radicals. These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O3. Most of the OHxe2x80xa2 radicals are produced in chain reactions where OHxe2x80xa2 itself or HO2xe2x80xa2 act as initiators.
Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Recent ozone research has been focused primarily on methods of using ozone, as discussed above, or methods of increasing the efficiency of ozone generation. For example, research in the electrochemical production of ozone has resulted in improved catalysts, membrane and electrode assemblies, flowfields and bipolar plates and the like. These efforts have been instrumental in making the electrochemical production of ozone a reliable and economical technology that is ready to be taken out of the laboratory and placed into commercial applications.
However, because ozone has a very short life in the gaseous form, and an even shorter life when dissolved in water, it is preferably generated in close proximity to where the ozone will be consumed. Traditionally, ozone is generated at a rate that is substantially equal to the rate of consumption since conventional generation systems do not lend themselves to ozone storage. Ozone may b stored as a compressed gas, but when generated using corona systems the pressure of the output gas stream is essentially at atmospheric pressure. Therefore, additional hardware for compression of the gas is required, which in itself reduces the ozone concentration through thermal degradation. Ozone may also be dissolved in liquids such as water but this process generally requires additional equipment to introduce the ozone gas into the liquid, and at atmospheric pressure and ambient temperature only a small amount of ozone may be dissolved in water.
Because so many of the present applications have the need for relatively small amounts of ozone, it is generally not cost effective to use conventional ozone generation systems such as corona discharge. Furthermore, since many applications require either ozone gas to be delivered under pressure or ozone dissolved in water as for disinfection, sterilization, treatment of contaminants, etc., the additional support equipment required to compress and/or dissolve the ozone into the water stream further increases system costs. Also, in some applications, it is necessary to maximize the amount of dissolved ozone in pure water by engaging ozone gas in chilled water under pressure. This mode of operation can minimize the amount of pure water required to dissolve a large amount of ozone. Such highly concentrated aqueous solutions of ozone can be added to a stream of process water to maintain a desired concentration of ozone in the process water stream.
Therefore, there is a need for an ozone generator system that operates efficiently on standard AC or DC electricity and water to deliver a reliable stream of ozone gas that is generated under pressure for direct use in a given application. Still other applications would benefit from a stream of highly concentrated ozone that is already dissolved in water where it may be used directly or diluted into a process stream so that a target ozone concentration may be achieved. It would be desirable if the ozone generator system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system had a minimum number of moving or wearing components, a minimal control system, and was compatible with low voltage power sources such as solar cell arrays, vehicle electrical systems, or battery power.
The present invention provides an ozone generating and delivery system that includes one or more electrolytic cells comprising an anode and a cathode. The system also includes an anode reservoir in fluid communications with the anode. The anode reservoir may comprise a water inlet and outlet port(s) for filling the reservoir with fresh water and discharging ozone saturated water. The anode reservoir may comprise a hydrophobic membrane at the top of the reservoir to allow ozone and oxygen gas to escape the anode reservoir while water is retained within the reservoir. The anode reservoir may be in thermal communication with a cooling member, such as a thermoelectric device, mechanical refrigeration unit or heat sink, for removing waste heat from the system. The anode is preferably in direct contact with the water in the anode reservoir allowing the free exchange of water with the reservoir and the transmission of gas from the anode to the anode reservoir. A water source providing deionized, reverse osmosis, distilled or other suitable water supply may be placed in fluid communication with the anode reservoir, preferably through a backflow prevention device. Alternatively, the anode may be operated in a self pressurizing mode so that when the anode pressure is momentarily relieved, the pressure of the water source is allowed to overcome the anode pressure and fill the anode reservoir with water, after which the anode relief is closed and the anode is again self pressurized through the generation of gas. The anode reservoir pressure may be held above the pressure of the water source by using a backflow prevention device or valve between the water source and the anode reservoir. In this manner, the pressure within the anode reservoir may be elevated to any desired pressure up to the design pressure of the hardware.
The ozone generator system may comprise: one or more electrolytic cells comprising of an anode and cathode; a power supply electronically coupled to the electrolytic cells; a battery back-up to the electrolytic cells to improve the lifetime of the anode electrocatalyst and provide rapid response to ozone demand; an anode reservoir in fluid communication with the anode and an anode gas releasing mechanism consisting of a porous hydrophobic membrane; a cathode in fluid communication with a cathode gas releasing mechanism consisting of a porous hydrophobic membrane; a recycle line for returning cathode water to the anode; and a cooling member for removing waste heat from the system.
Another aspect of the invention provides a waste gas destruction system which utilizes a catalyst to combine the hydrogen with oxygen from the air to consume the hydrogen without a flame and generate waste heat. In addition to other processes which may utilize this high-grade, contaminant free, waste heat, this hydrogen destruct system may be in thermal communication with an ozone destruction system comprising of a catalyst suitable for the conversion of ozone into diatomic oxygen.
In another aspect of the invention, a process for generating and delivering ozone is provided comprising the steps of: electrolyzing water in one or more electrolytic cells to generate a combination of oxygen and ozone at the anode and hydrogen at the cathode; utilizing a natural means of circulation, such as gas lift, gas forced and thermal, to circulate water between reservoirs and the electrolytic cells; separating the ozone/oxygen gas from the anode water using a porous hydrophobic membrane; receiving hydrogen gas and water from the cathode; phase separating the hydrogen from the cathode water; returning the water originally transferred from the anode to the cathode through electroosmosis back to the anode; separating and discharging the hydrogen gas using a porous hydrophobic membrane which eliminates the requirements for mechanical valves or a control system; adding water to the anode on a continuous or periodic basis to maintain the water supply, self pressurizing the system allowing the delivery of a pressurized oxygen/ozone, hydrogen, and oxygen/ozone saturated water. Other beneficial steps may be taken, including: operating the system at elevated pressures to dissolved higher levels of ozone into solution, and to deliver ozone gas and ozonated water under pressure to eliminate further pumping; removing waste heat from the system and lowering the system temperature to dissolve more ozone in the water and increase the ozone lifetime; destroying the surplus ozone and hydrogen so that the system may be operated in an enclosed environment without necessitating venting; using the waste heat from the hydrogen destruction to enhance the catalytic destruction of the ozone; and/or utilizing the high grade waste heat from the entire gas destruct unit to provide heating to another process.